New Haven, Connecticut (CNN) -- A man convicted of killing a mother and her two daughters during a 2007 Connecticut home invasion can be "quite likable," but has struggled with drug addiction for years, a defense attorney told jurors Monday.
The same jurors who convicted Steven Hayes of capital murder and other charges convened Monday for Hayes' penalty phase. They will decide whether he should be sentenced to death for the slayings of Jennifer Hawke-Petit and her daughters, 17-year-old Hayley Petit and 11-year-old Michaela Petit.
Attorney Patrick Culligan told jurors in his opening statement they will hear testimony from "people who know Steve Hayes, whose lives have crossed paths with Steven Hayes ... to try to give you some insight, some understanding of who Steven Hayes was prior to the crimes of July 23, 2007." Some of the testimony, he warned, will not be flattering to Hayes, but he said that despite his long history of criminal convictions and drug abuse, Hayes could be "quite likable."
"There is no presumption at this point that the death penalty is the appropriate punishment," Culligan said. "... You must keep an open mind."
Culligan said testimony would also focus on Hayes' behavior in prison in the past, noting that he has been incarcerated for "most of his adult life" because of his addiction.
"At the conclusion [of testimony], you'll come to know much more of the 'who' about Steven Hayes," Culligan told jurors. "... Hopefully, you'll also have a fuller understanding of his role in these crimes ... these most horrible and tragic crimes."
The first witness for the defense, social worker D'Arcy Lovetere, crossed paths with Hayes many times in her role working for the public defender's office.
"What stands out about Steven when he would get arrested was how desperately he wanted to stop the behavior," she said.
"He really wanted a life. ... He knew he needed substance abuse treatment."
Lovetere said Hayes "seemed to be struggling" when she saw him shortly before the home invasion. She recalled her shock at hearing of his arrest.
"It knocked the wind out of me. I just couldn't believe it," she said.
Hayes, 47, was convicted earlier this month of 16 of the 17 charges against him, including nine counts of murder and capital murder and four counts of kidnapping. Jurors acquitted him of an arson charge.
During the penalty phase, jurors will determine whether the mitigating evidence the defense is expected to present outweighs the aggravating factors in favor of the death penalty.
Jurors are allowed to consider evidence they heard during the guilt phase of the trial. They can also consider new testimony or evidence that attorneys introduce during the sentencing phase, which might include the defendant's long history of prior convictions and anything that casts Hayes in a positive light. Hayes himself could testify.
"All of the evidence presented in the guilt phase may be considered in this phase ... and the state is relying on that evidence," prosecutor Michael Dearington told jurors in his opening statement.
The only new testimony presented in the penalty phase by the prosecution concerned Hayes' prior convictions -- two in 1981 and others in 1993, 1996 and 2003, all for third-degree burglary. The prosecution rested just before the court took a lunch break on Monday.
Throughout the day, the defense continued to call witnesses who had interacted with Steven through work or from his apartment complex or because of his drug addiction.
Another social worker who met Hayes through his prior run-ins with the law testified that Hayes never seemed violent, but was "fragile."
"When people don't resolve their emotions, you allow that emotion to control you," Sylvester Esangbedo said. "Anger can become rage. He allowed his anger to control him, because he was unable to forgive himself."
The killings took place in the New Haven suburb of Cheshire. They shocked the well-to-do community and drew nationwide attention.
The home of Dr. William Petit was invaded in the middle of the night by Hayes and another man, Joshua Komisarjevsky, prosecutors say. Komisarjevsky will be tried separately.
Connecticut State Police Detective Anthony Buglione, who interviewed Hayes after the crime, has testified that the duo beat Petit bloody and left him in the basement.
According to testimony, the two men then went upstairs and found Hawke-Petit and Michaela asleep in the master bedroom. After tying Hawke-Petit to her bed, they led the girl to her room, tied her to her bed and put a pillowcase over her head, Hayes told Buglione. They then found Hayley Petit in her room and did the same, he said.
After finding evidence of a bank account containing $20,000 to $30,000, they decided to have the mother go to the bank in the morning and withdraw money from her account, Buglione testified.
Hayes is accused of taking Hawke-Petit to the bank while Komisarjevsky allegedly stayed behind. When Hayes and Hawke-Petit returned with the money, the two men allegedly set the home on fire and fled.
Inside the home, authorities said, Hawke-Petit, 48, was found raped and strangled. Her two daughters, one of whom had been sexually assaulted, died of smoke inhalation. Petit, the sole survivor, escaped to a neighbor's home.
William Petit has said he will not offer a victim impact statement during Hayes' penalty phase, saying in a statement he "regretfully" decided against doing so because Connecticut's law on victim impact statements is unclear and could provide convicts with grounds to appeal their sentences.
Culligan noted in his remarks to jurors Monday that "There was another person involved in this crime: Joshua Komisarjevsky." He said jurors will hear about Komisarjevsky in the suspect's own words, much of it from his writings to author Brian McDonald.
Judge Jon Blue has suggested the penalty proceeding could last as long as four weeks, though the relatively quick pace of the guilt phase of Hayes' trial has led some to speculate the penalty phase could also be quick.
Hayes' attorneys had sought to introduce testimony regarding the comparative cost of life imprisonment versus execution, but Blue barred the subject.
Proceedings are expected to continue Tuesday.